>Interesting,
>this description fits in on the 4CX350AC, which is marketed by Svetlana
>and others
>as a lower temperature cathode and consequently longer life version of the
>4CX350A.
>
>I have encountered the 4CX250BC version of the 4CX250B, which should have
>similar
>cathode properties. As far as I recall, the 4CX250BC was tried as one
>possible cure for the
>tube life problems in the SRT ST1610 transmitters, but only with moderate
>success.
>
>The 4CX350AC was considered for our Collins 208U-10's, but the solid-state
>driver chain
>modifications were done instead.
>
>It appears that tube manufacturer application engineers caution you
>against running oxide cathodes both undervoltage
>and overvoltage, overvoltage seems also to speed up the malignant type of
>grid contamination that causes grid emission and thermal
>runaway besides long term loss of emission,
>but I cannot recall seeing any firm statements on the ill effects on
>undervoltage, except low initial emission.
>
>In the "old days" of UHF Class C amplifiers it was common to derate the
>oxide cathode filament voltage to compensate
>for RF losses and electron back-bombardment of the cathode and a
>consequent increase in surface temperature.
*** I've heard of cases in the 2GHz range where it was found that
back-bombardment was such that the heater potential could be removed
shortly after nominal RF output was achieved -- i.e., the cathode
temperature was being maintained by the RF.
>
>...
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